Assuming you registered your installation of MM when you first installed it you need to go to your Magix account, log in, go to My products and find the listing for your software. You should find a download link and your serial key listed there.
If you cannot access your account you will need to email infoservice@magix.net, explaining what you want and giving as much detail as possible about the original purchase of the software.
@David-Huffman Between Music Maker 2018 and 2024 the base app was free, so maybe it's not showing because of this.
If you bought Music Maker 2019 inside the in-app-store it might not have a separate serial number. I would suggest you try this download link and install it. This should be the link for Music Maker 2024. After you run the program and login into your account it should automatically recognize your version 2019 and unlock the features.
By the way, there is still three hours at Humble Bundle left where you can get Music Maker 2024 Premium plus additional software for $25. Maybe it's worth the upgrade: